The contents of Japanese Patent Applications Nos. 11-245390, with a filing date of Aug. 31, 1999, in Japan, and 11-262260, with a filing date of Sep. 16, 1999, in Japan, are hereby incorporated by reference.
The present invention relates to a technique for controlling a spark-ignited direct fuel injection internal combustion engine.
Recently, the technique of direct fuel injection in a spark ignition engine is under development to improve fuel efficiency and emission control by injecting fuel directly into a combustion chamber (or each combustion chamber). A control system for such a type normally operates combustion of a homogeneous air fuel mixture distributed uniformly throughout the combustion chamber by fuel injection on the intake stroke. In a predetermined engine operating condition (such as a condition of low revolution speed and low load), the control system operates with ultra lean combustion by injecting fuel on the compression stroke so as to produce an ignitable stratified air fuel mixture around an ignition plug.
Japanese Patent Provisional (Kokai) Publications Nos. 62(1987)-191622 and 2(1990)-169834 disclose internal combustion engines of such a type.
Japanese Patent Provisional (Kokai) Publications No. 10(1998)-169488 further discloses a related technology for activating a catalytic converter. According to this technology, there is produced the stratified air fuel mixture that is richer than the stoichiometric air fuel ratio (i.e. theoretical air fuel ratio) locally around the ignition plug so as to produce a region wanting the oxygen in the combustion chamber. The combustion of the stratified air fuel mixture produces the products of incomplete combustion (such as CO) in this region and raises exhaust gas temperature by means of reacting the product of incomplete combustion (CO) and unburned fuel with the oxygen in the background of the oxygen-lacking region after a main combustion (i.e. combustion by spark ignition and flame propagation thereby).
In the technology mentioned above, the homogeneous combustion is necessary at first to start the engine that has not finished its warm-up for achieving the stable combustibility, and then the stratified combustion is operated according to a request for raising the exhaust gas temperature. After the catalytic converter has been activated, the stratified combustion is converted to the homogeneous lean combustion, which is further converted to the stratified lean combustion, or the homogeneous stoichiometric combustion according to the engine operating condition.
However, the stratified combustion has lower thermal efficiency than the homogeneous combustion in which the air fuel mixture is sufficiently mixed throughout the combustion chamber, therefore there is an undesired sudden change of the engine torque (that is referred to as xe2x80x98the torque differentiationxe2x80x99 later) when the stratified combustion is converted to the homogeneous combustion, and vice versa. Such torque differentiation may impair the drivability.
Therefore, an object of the invention is to provide a spark-ignited direct fuel injection internal combustion engine that can reduce the torque differentiation mentioned above when a combustion mode is converted according to the engine operating condition.
The above and other objects of the present invention can be accomplished by a control system for an internal combustion engine that comprises an internal combustion engine having a combustion chamber, an injector provided for the combustion chamber to directly inject fuel and produce an air fuel mixture therein, and an ignition plug provided for the combustion chamber to ignite the air fuel mixture. The control system also comprises a controller for operating the internal combustion engine in one of combustion modes selectively between a homogeneous combustion mode and a stratified combustion mode in response to a request according to an engine operating condition. The controller further adjusts a parameter that affects engine torque of the internal combustion engine so as to reduce the torque differentiation when the combustion mode is converted.
The controller may operate the stratified combustion mode by controlling fuel injection quantity, fuel injection timing, and ignition timing so as to produce and combust a stratified air fuel mixture that is of richer-than-stoichiometric air fuel ratio around the ignition plug, operate the homogeneous combustion mode by controlling the fuel injection quantity, the fuel injection timing, and the ignition timing so as to produce and combust homogeneous air fuel mixture that is distributed uniformly throughout the combustion chamber, convert the combustion mode between the stratified combustion mode and the homogeneous combustion mode upon occurrence of the request according to the engine operating condition, and adjust the parameter so as to reduce the torque divergence when the combustion mode is converted.
If there is the request for raising the exhaust gas temperature, the controller produces and combusts the stratified air fuel mixture that is richer than stoichiometric ratio around the ignition plug (by controlling the fuel injection quantity, the fuel injection timing, and the ignition timing by operating the stratified combustion mode). Because of the richer-than-stoichiometric stratified air fuel mixture, the products of incomplete combustion (such as CO) is produced by the main combustion. The product of incomplete combustion is combusted in the combustion chamber and/or an exhaust passage after the main combustion, therefore the controller can raise the exhaust gas temperature and then promote an activation of an exhaust emission control device such as a catalytic converter.
On the other hand, there is generated a request to operate the homogeneous combustion mode in such engine operating conditions that the engine has not finished its warm-up or the exhaust emission control device has been activated. In such conditions, the controller produces and combusts the air fuel mixture distributed uniformly throughout the combustion chamber (by controlling the fuel injection quantity, the fuel injection timing, and the ignition timing by operating the homogeneous combustion mode).
The controller adjusts the parameter that affects the engine torque so as to reduce the torque differentiation (by adjusting the parameter mentioned above) when the combustion mode is converted according to a change of the request between the homogeneous combustion mode and the stratified combustion mode.
In this manner, the torque differentiation can be reduced, and stable drivability can be achieved.
An exhaust system of the control system may comprise an exhaust passage connected to the internal combustion engine and an exhaust emission control device disposed in the exhaust passage. To generate the request for operating the stratified combustion mode, the controller may check activeness or inactiveness of the exhaust emission control device and permit the stratified combustion mode when the exhaust emission control device is inactive.
In operating the stratified combustion mode, it is desirable that an average air fuel ratio in the entirety of the combustion chamber is set at the stoichiometric ratio to achieve the desired emission performance by the emission control device. Especially when the average air fuel ratio is set substantially equal to the stoichiometric ratio, the amounts of CO and residual oxygen are approximately equivalent and the temperature of the exhaust emission control device increases very efficiently.
The stratified combustion mode may be operated by dividing a fuel injection into a first portion injected on an intake stroke for producing a leaner-than-stoichiometric air fuel mixture throughout the combustion chamber and a second portion injected on a compression stroke for producing a richer-than-stoichiometric air fuel mixture around the ignition plug, and combusting the air fuel mixture produced in the combustion chamber.
In this stratified combustion mode, the richer-than-stoichiometric air fuel mixture around the ignition plug combusts in a main combustion (i.e. combustion by spark ignition and flame propagation thereby) and produces carbon monoxide (CO) due to the incomplete combustion. The carbon monoxide is re-combusted with the leaner-than-stoichiometric air fuel mixture in the background of the stratified air fuel mixture. Propagation of re-combusting flame over the combustion chamber reduces a low temperature region (quenched region) to a sufficiently small extent as in the homogeneous combustion mode. Because the lean background region acts to leave behind an excess of oxygen after the main combustion, the residual oxygen in the lean region is so hot as to promote the re-combustion of CO at the end of the main combustion.
Preferably, the fuel injection quantity of the first portion is equal to or larger than that of the second portion for keeping a balance between the amount of the products of incomplete combustion (such as CO) and the amount of oxygen left behind after the main combustion to effectively increase the temperature of the catalytic converter.
According to the present invention, the parameter that affects the engine torque can be the ignition timing, the fuel injection timing, and the fuel injection quantity, for example.
It is desirable that the ignition timing while the stratified combustion mode is maintained (i.e. the target ignition timing in the stratified combustion mode) be retarded more than the ignition timing while the homogeneous combustion mode is maintained in order for increasing the product of incomplete combustion and thus raising the exhaust gas temperature.
In the case of adjusting the ignition timing as the parameter that affects the engine torque, the controller may gradually adjust said ignition timing from an occurrence of a request for converting said combustion mode until said combustion mode is converted in one direction, adjust said ignition timing in the other direction to reduce said change of said engine torque at the same time the combustion mode is converted, and gradually adjust said ignition timing after said combustion mode is converted in the same direction before said combustion mode is converted.
When the controller converts the homogeneous combustion mode to the stratified stoichiometric combustion mode, there is a request for increasing the engine torque in order to prevent the torque differentiation caused by converting to the stratified stoichiometric combustion mode that is of lower thermal efficiency than the homogeneous combustion mode. However, on operating the homogeneous combustion mode, the ignition timing is controlled to keep MBT (i.e. the Minimum spark advance of Best Torque) to achieve the desired fuel economy (or the engine stability), therefore the ignition timing cannot be advanced any more to increase the engine torque.
On the other hand, when the controller converts the stratified stoichiometric combustion mode to the homogeneous combustion mode, there is a request for decreasing the engine torque in order to prevent the torque differentiation caused by converting to the homogeneous combustion mode that is of higher thermal efficiency than the stratified stoichiometric combustion mode. However, on operating the stratified stoichiometric combustion mode, the ignition timing is controlled as much retarded as the engine can maintain the stable operation for promoting atomization and vaporization of the stratified air fuel mixture, therefore the operation of the engine becomes unstable if the ignition timing is retarded anymore, and it is substantially impossible to adjust the ignition timing in the retarding direction any more.
For this reason, in the case that the combustion mode is converted from the homogeneous combustion mode to the stratified combustion mode, the controller gradually retards the ignition timing before the combustion mode is converted. And in the case that the combustion mode is converted from the stratified combustion mode to the homogeneous combustion mode, the controller gradually advances the ignition timing before the combustion mode is converted. After the combustion mode is converted, the ignition timing is adjusted at the same time, in the opposite direction to the gradual adjustment mentioned above, in order to reduce the torque differentiation, and the ignition timing is gradually adjusted to the target amount in the present combustion mode in the same direction as before the combustion mode is converted.
In this manner, the torque differentiation can be sufficiently reduced, and the engine torque before and after the combustion mode is converted can be changed gradually.
Furthermore, the controller may adjust intake airflow rate to reduce a change of the engine torque caused by gradually adjusting the ignition timing. When the ignition timing gradually retards, the engine torque changes in the decreasing direction, thus the intake airflow rate is adjusted in the increasing direction to reduce the amount of decrease of the engine torque, and when the ignition timing gradually advances, the engine torque changes in the increasing direction, thus the intake airflow rate is adjusted in the decreasing direction to reduce the amount of increase of the engine torque.
In this manner, the change of the engine torque can be reduced as much as possible throughout the process from the time when the controller permits conversion of the combustion mode until the ignition timing converges on the target ignition timing and the engine is brought into the stationary state after the combustion mode has been converted.
To adjust the intake air flow rate, an intake system of the internal combustion engine may comprise an intake passage connected to the internal combustion engine, a throttle valve disposed in the intake passage for controlling intake airflow rate, a throttle controller for electrically controlling the opening degree of the throttle valve, and for adjusting the intake airflow rate in response to the controller gradually adjusting the ignition timing.
On operating the stratified combustion mode, fuel injection quantity ratio of the first portion to the second portion may be set to be increased near an engine operating condition in which a conversion to the homogeneous combustion mode is permitted, so that the fuel injection quantity ratio is set larger relative to being in the engine operating condition far from which the conversion to the homogeneous combustion mode is permitted.
In this manner, the torque differentiation can be reduced because the ratio of the homogeneous air fuel mixture to the whole in the combustion chamber increases, and the combustion mode can be converted to the homogeneous combustion mode from the stratified combustion mode that increases its homogeneity. Therefore it makes it easy to convert the combustion mode, and makes it possible to shorten the time that it takes to gradually adjust the ignition timing before and/or after the combustion mode is converted so that the combustion mode can be converted quickly.
In the case of adjusting the fuel ignition timing as the parameter that affects the engine torque, it is easy to decrease the torque differentiation. Specifically, the controller may gradually advance the fuel injection timing from an occurrence of the request until the combustion mode has converted in a case of converting the stratified combustion mode to the homogeneous combustion mode, and in a case of converting the homogeneous combustion mode to the stratified combustion mode, adjusts the fuel injection timing in the advancing direction when the combustion mode is converted, then gradually retards the fuel injection timing.
When the stratified combustion mode is converted to the homogeneous combustion mode, the controller gradually advances the fuel injection timing on the compression stroke, so that the intake air quantity that can be mixed with fuel in the combustion chamber increases, hence the degree of homogeneity of the air fuel mixture increases. The higher the degree of homogeneity of the stratified combustion, the closer the stratified combustion mode comes to the homogeneous combustion mode, and the more the engine torque increases. Therefore the controller can reduce the torque differentiation and bring the engine torque to change gradually before the conversion of the combustion mode because the combustion mode is converted after the engine torque is gradually increased.
In the case of adjusting the fuel ignition quantity as the parameter that affects the engine torque, it can easily decrease the torque differentiation. Specifically, in a case of converting the stratified combustion mode to the homogeneous combustion mode, the controller gradually increases the fuel injection quantity from an occurrence of the request until the combustion mode has been converted, then adjusts the fuel injection quantity in the decreasing direction to an amount without adjustment when the combustion mode is converted, and in a case of converting the homogeneous combustion mode to the stratified combustion mode, adjusts the fuel injection quantity in the increasing direction when the combustion mode is converted, then gradually decreases the fuel injection quantity to an amount without adjustment.
In this manner, when the stratified combustion mode is converted to the homogeneous combustion mode, the controller gradually increases the fuel injection quantity, so that the engine torque can gradually increase before the combustion mode is converted. Therefore the controller can reduce the torque differentiation and bring the engine torque to change gradually before the combustion mode is converted.
In the present invention, as far as only the reduction of the torque differentiation is concerned, the controller may adjust the fuel injection quantity only once when it converts the combustion mode. However, the controller may gradually adjust the fuel injection quantity before or after the combustion mode is converted (as mentioned above), so that the controller can control the fuel injection quantity at the same amount between the homogeneous combustion mode and stratified stoichiometric combustion mode at the same engine operating condition and the air fuel ratio can be maintained the stoichiometric ratio (xcex=1) in order to maintain the performance of purifying the exhaust gas.
In adjusting the fuel injection quantity, it may be the fuel injection quantity on the intake stroke and/or on the compression stroke that is gradually adjusted in the stratified stoichiometric combustion mode.
Furthermore, it may be the fuel injection quantity in the homogeneous combustion mode that the controller adjusts. Specifically, the controller adjusts the fuel injection quantity in the decreasing direction when the combustion mode is converted, then gradually increases the fuel injection quantity to an amount without adjustment, in a case of converting the stratified combustion mode to the homogeneous combustion mode, and in a case of converting the homogeneous combustion mode to the stratified combustion mode, gradually decreases the fuel injection quantity from an occurrence of the request until the combustion mode has converted.
In the manner mentioned above, when the controller converts the homogeneous combustion mode to the stratified stoichiometric combustion mode, the controller gradually adjusts the fuel injection quantity in the decreasing direction from the amount determined from the engine operating conditions by an amount so that the controller can reduce the torque differentiation when the combustion mode is converted, and then it converts to the stratified stoichiometric combustion mode. The fuel injection quantity is controlled according to the engine operating conditions in the stratified stoichiometric combustion mode when and after the combustion mode is converted thereto.
In converting the stratified stoichiometric combustion mode to the homogeneous combustion mode, the controller takes the process described above in the opposite direction. That is, just after the combustion mode is converted to the homogeneous combustion mode, the controller decreases the fuel injection quantity at once by an amount such that the torque differentiation can be canceled, and then, it gradually increases the fuel injection quantity to the amount determined from the engine operating condition in the homogeneous combustion mode (for example, the amount by which the air fuel ratio can be maintained at the theoretical air fuel ratio (xcex=1)).
In this manner, it is also possible to reduce the torque differentiation when the combustion mode is converted, by decreasing the fuel injection quantity in the homogeneous combustion mode.
Further objects, features and advantages of the present invention will become apparent from the Detailed Description of Preferred Embodiments which follows when read in light of the accompanying figures.